So summer is over and we're back working on the car. :-)
The break issue is still there. After the car gets warm, the front breaks will get hot (and when you slow down will actually stop the car without power), but only sometimes.

I have replaced the flex lines on both sides, the calipers on both sides, bled all the breaks enough to have replaced all the fluid, and just did the master cylinder today. After about 7 miles of driving the problem reappeared. :-(

I think all that's left is the booster and the proportioning block.

Any suggestions? Am I overlooking something obvious?

 

did you replace the pins/slides when you did new calipers?

 

Here is a test to do. Get the brakes as hot as you can so they are holding the car back, then quickly open the bleeder. If the brake fluid shoots out you have a hydralic issue if not you have a mechanical issue are the brakes themself. You want to check both side. Knowing if it is one side or both sides would help to narrow the issue down because the front rotors are seperate hydralic circuits. If its at both them it may be coming from the master cylinder area.

At the garage we would install an inline pressure gage to see what was going on. I can not say I ever remember needing the gage for Brakes sticking problem. We only used it for uneven braking issue. I did have one odd ball issue but that was with a cutomer that Converted a car to power brakes. The brake rod was just a tad too long and when the engine bay got hot it would apply the brakes a small amount.

 

The brake lines or should I say brake flex hoses can be a tough one if you don't know what to look for.

 

I replaced the pins/slides/bolts when I did the calipers. The breaking is even on both sides, and when its hot, I can open the bleeder and the brakes release immediately. So, that sounds like a hydraulic issue doesn't it?

There is some corrosion at the front of the booster (where it contacts the master cylinder). Can a booster cause this problem? For that matter, what can?

 

You could do the same test with the booster. Get the brakes hot then loosen the two bolts that hold the master cylinder to the booster. If the brakes release then its a bad booster or wrong push rod or wrong master cylinder. If it doent loosen then it is something between the Master cylinder and Calipers. After that start craking lines to see where there is not pressure when hot.

 

Just a couple of thoughts...

Has the car had any of the hard brake lines replaced? By asking this, is it possible that the master cylinder ports for the front & rear brakes were accidentally plumbed in reverse?

Next question, have you looked close at the proportioning valve? There is a bypass feature that insures full pressure to the rear brakes should the front system fail.

If this bypass is faulty, remember the rear drums always have a small residual pressure maintained to keep the wheel cylinders snug against the shoes, where the front discs are designed to sit with zero residual pressure.

Are the brake stop lamp switch & cruise control vacuum dump correctly adjusted or are the switch bodies just touching the brake pedal arm?